To Daisy, this was a perfectly reasonable explanation of the wealth of her new neighbor, Jay Gatsby. To her husband, more knowing about the world beyond the boundaries of East Egg, it was evidence that Gatsby had made his money as a bootlegger.

Modern readers in the grip of F. Scott Fitzgerald's prose may not recognize the meaning of Tom Buchanan's insight, but Fitzgerald knew his contemporaries would understand. In 1925, when The Great Gatsby was published, the meaning of "drugstores" was as clear as gin: Those were the places you went to get medically prescribed alcohol, a legally acceptable source of liquor during all 13 years of Prohibition.

Sound familiar? To any modern Californian, of course it does.

For most of the 1920s, a patient could get a prescription for one pint every 10 days about as easily as California patients can now get "recommendations" for medical marijuana. All it took to acquire a liquor prescription was cash — generally about $3, the equivalent of about $40 today — placed in the hand of an agreeable doctor. It cost $3 to $4 more to have it filled by the local pharmacist.

Then as now, the adaptability of the medical profession was impressive. In 1917, as the 18th Amendment establishing Prohibition was working its way through the ratification process, the American Medical Association ousted alcohol from its approved pharmacopoeia, adopting a unanimous resolution asserting that its "use in therapeutics ... has no scientific value."

But the Volstead Act, which spelled out the enforcement and regulation of Prohibition, nonetheless made an exception for medicinal use, and in 1922, just two years into the dry era, the AMA demonstrated how open minds can be changed — or, perhaps, how capitalism abhors a missed opportunity.

The results of a national survey of its members — a "Referendum on the Use of Alcohol in the Medical Profession" — revealed an extraordinary coincidence: The booming prescription trade had been accompanied by the dawning realization among America's doctors that alcoholic beverages were in fact useful in treating 27 separate conditions, including diabetes, cancer, asthma, dyspepsia, snake bite, lactation problems and old age. In a word, the assertion that medicinal alcohol had "no scientific value," from the AMA's 1917 resolution, no longer had any scientific value.

Pharmacists who wanted a piece of this highly profitable new business devised practices appropriate to their clientele. Those with high-end customers, mindful of the power (and profit) in brand names, dispensed the prescribed "medicine" in the distillers' own bottles, which looked exactly as they had before 1920 except for the addition of a sober qualifying phrase on their newly printed labels: A 100-proof pint of Old Grand-Dad, for instance, still announced that it was "Bottled in Bond," but just beneath that familiar legend appeared the improbable phrase, "Unexcelled for Medicinal Purposes."

In Chicago, druggist Charles Walgreen saw his chain expand from 20 stores in 1920 to a staggering 525 a decade later. Along the way, Walgreen's introduced the milkshake, which family historians have credited with the chain's rocketing expansion. But it's doubtful that milkshakes alone were responsible. Something Charles Walgreen Jr. told an interviewer many years later suggests another possibility. The elder Walgreen worried about fire breaking out in his stores, his son recalled, but this apprehension extended beyond an understandable concern for the safety of his employees: He "wanted the fire department to get in as fast as possible and get out as fast as possible," Charles Jr. remembered, "because whenever they came in, we'd always lose a case of liquor from the back."

All that "medicinal" whiskey (and rum and gin and brandy and every other imaginable liquid intoxicant) was perfectly legal. But it also made a mockery of the law, debased the dignity of the medical profession and encouraged rampant criminality, as mobsters eventually and inevitably took over much of the medicinal market. What finally straightened out the liquor business was the legalization that came with repeal in 1933 — legalization that was accompanied by a coherent and effective set of enforcement laws, a healthy boost in tax revenues (in the first postrepeal year, the federal government was enriched by the 2010 equivalent of $4 billion in alcohol tax revenue), and an honest recognition that, all too often, "medicinal" had been a cynical euphemism for "available."

Daniel Okrent is the author of "Last Call: The Rise and Fall of Prohibition."